The present invention relates to a semiconductor circuit including field effect transistors (hereinafter referred to as "FET's"), and more particularly to a semiconductor circuit whose electrical characteristics are independent of the threshold voltage of FET's, included in an integrated circuit (hereinafter referred to as "IC"), making up the semiconductor circuit. Further, the present invention relates to a signal processing system using the above semiconductor circuit.
A current source circuit using FET's is explained on pages 709 through 718 of a text book entitled "Analysis and Design of Analog Integrated Circuit" second Edition, published in 1984 by John Wiley & Sons, Inc.
A current source circuit of this kind is called "current mirror circuit", and has first and second FET's and input, common and output nodes. In more detail, the drain-source current path of the first FET is connected between the input node and the common node, the drain-source current path of the second FET is connected between the output node and the common node, and the gate of the first FET and the gate of the second FET are connected to each other, to be shorted to the drain of the first FET.
Further, each of the first and second FET's is of the enhancement type, and the gate of the first FET is shorted to the drain thereof. When the gate-source voltage of the first FET exceeds the threshold voltage thereof, the first FET operates in a saturation region. Further, when a voltage applied between the output node and the common node is sufficiently high, the second FET operates in a saturation region.
An input current to the above current mirror circuit flows through the drain-source current path of the first FET, to generate the gate-source voltage of the first FET. This gate-source voltage is applied between the gate and source of the second FET. Thus, in a case where the second FET is equal in threshold voltage and effective device area to the first FET, an output current equal to the input current flows through the drain-source current path of the second FET.
Further, in a case where the effective device area of the second FET is twice larger than that of the first FET, the output current is twice larger than the input current.
As mentioned above, when a predetermined input current is supplied to the input node of the current mirror circuit, an output current proportional to the input current flows through the output node. This operation is analogous to the reflection of light from a mirror. Hence, a current source circuit of this kind is called "current mirror circuit".